This invention relates to an apparatus for coupling radio frequency energy to conductive surfaces for radiation and, more specifically, providing the coupling by current injection.
A conventional antenna utilizes current being developed on its structure to generate a radiated radio frequency electric field. Conventional antennas are available in many forms and sizes. The lower the frequency of transmission, the longer the antenna required to properly develop a usable radiated power. Therefore, in some low frequency applications, the antenna becomes large, perhaps difficult to mount to a nearby structure, and may require a significant outlay of funds for the purchase of a proper antenna and its mounting hardware.
One type of antenna is a form of wire or conductor arranged in a linear configuration, such as a metallic rod. If the current distribution on such a conductor is known, then the radiation pattern and radiated power can be determined. This determination is based upon the integration of the effects from each differential element of current along the conductor.
A commonly-accepted expression for the electric field generated at large distances from a relatively short (less than a quarter wave length of the radiated signal) vertically polarized antenna is,                               E          θ                =                  j          ⁢                      xe2x80x83                    ⁢                                    60              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢              sin              ⁢                              xe2x80x83                            ⁢              θ                                      r              ⁢                              xe2x80x83                            ⁢              λ                                ⁢                      I            0                    ⁢                      L            e                    ⁢                      ⅇ                                          -                j                            ⁢                              xe2x80x83                            ⁢              β              ⁢                              xe2x80x83                            ⁢              r                                                          (        1        )            
where:
Le=effective length of antenna
I0=Current at center of antenna in amperes (RMS)
r=distance in meters to the observation point
xcex=wavelength (meters)
xcex2=2xcfx80/xcex
Excex8=volts/meter
xcex8=angular position of the observation point
j={square root over (xe2x88x921)}
This expression demonstrates that any conductor developing an RF current can be considered an antenna and the field generated by a conductor of any length is a function of the current generated in the conductor.
The ability of an antenna or metallic structure to radiate is a function of its radiation resistance. The radiation resistance, along with the current on the antenna, is responsible for generating RF power radiating outward from the antenna.
The radiation resistance for a monopole-type antenna, such as a linear metal conductor, that is less than a quarter wavelength is commonly given by:                               R          r                =                  40          ⁢                      xe2x80x83                    ⁢                                                    π                2                            ⁡                              (                                                      L                    e                                    λ                                )                                      2                                              (        2        )            
where, as in equation 1, Le is the effective height of the antenna or one half the physical length of the antenna. For example, a 0.1 wavelength monopole antenna will have a radiation resistance of at least 5 ohms, and 0.35 wavelength monopole will have a radiation resistance of approximately 50 ohms. Note that the radiation efficiency of a monopole antenna can be improved by xe2x80x9ctop-loadingxe2x80x9d the antenna, or placing a disk of metal or radial wires at one end of the antenna. The disk or wires add capacitance, and therefore impedance matches the antenna to a lower frequency than if the disk or wires were not present. This permits a greater current over a longer length of the antenna, thus improving the radiation efficiency.
The power radiated by a monopole type of antenna is given by
Pr=I2Rrxe2x80x83xe2x80x83(3)
where I=current in amperes.
The vertical electric field developed by an isotropic antenna, in terms of radiated power is commonly given by:                               E          θ                =                                                            30                ⁢                                  xe2x80x83                                ⁢                                  P                  r                                            r                                ⁢                      xe2x80x83                    ⁢                      volts/meter at the observation point                                              (        4        )            
From the above model, a monopole metallic structure 0.1 wavelengths long at 10 MHz, in theory, is physically 3 meters in length. The electrical effective height is approximately one half its physical length or 1.5 meters. Many metallic structures in today""s environment, not initially intended to be used as antennas, will meet this physical requirement. Such structures may include portions of a vehicle, a building, or a ship.
Direct coupling of RF energy to structures like those cited above is generally not feasible. Serious safety concerns arise when sources of large amounts of current are directly connected to metallic structures that may come into contact with human beings. Also, impedance mismatches between the source of the RF energy and the metallic structures are likely to result in most of the RF energy being reflected back to the source. In extreme cases, the reflected energy may damage the RF source.
Coupling of RF energy to conductors can also be accomplished by using a series capacitor. As is known in the art, this technique has been used to couple RF energy to an electrical power wire (such as electrical wiring in a house), which then acts as an antenna. The series capacitor presents a high impedance to the frequency of the electrical power carried by the wire, but presents a low impedance to the higher frequency RF energy. Thus, the RF energy can be coupled to the electrical power wire. This technique still requires a direct connection to the conductor, therefore retaining the safety and direct physical connection concerns.
In another field of technology, related only in the environment involving RF energy and associated RF electrical signals, are instrument transformers, or devices commonly referred to as RF current injection probes, which are well known. These devices are designed to be used in laboratory instrumentation applications for purposes of making measurements; that is, RF injection probes have been typically used to couple RF currents into the wires of a device under test. Such testing is typically required during electromagnetic interference susceptibility testing required by civil regulatory agencies like the Federal Communications Commission, the European Economic Union, and the Military when certifying a piece of equipment or confirming conformance to electromagnetic compatibility standards.
In U.S. Pat. No. 5,633,648, issued May 27, 1997, to the Applicant, an apparatus for utilizing instrument transformers, or RF current probes, for receipt of RF energy is disclosed. However, these instrument transformers are constructed so as to operate at lower currents than instrument transformers used for current injection. Furthermore, losses induced by instrument transformers used for receipt of RF energy are of less concern, since circuitry within the RF receiver coupled to the instrument transformer can be used to amplify the RF signal to an acceptable level for further processing. Therefore, instrument transformers constructed for use as RF current injection probes are constructed with limitation of transformer induced losses as a primary concern.
Known RF current injection probes generate RF current in any metallic wire, rod, or surface. Such RF current injection probes may be constructed in many different embodiments well known in the art. Two exemplary embodiments are described below.
A first embodiment of an RF current injection probe comprises a toroidal magnetic core and winding, which provide the primary winding of a transformer, and a single metallic wire, rod or other conductor passing through the aperture of the toroid, acting as a secondary winding. In this embodiment, the primary winding, when connected to an RF current source, will cause RF current flow to be induced in the secondary winding.
A second embodiment of an RF current injection probe is one half of a toroidal or rectangular magnetic core and a winding, representing the primary winding of a transformer. The end surface faces of the toroid core or rectangular core are electrically insulated and placed close to a metallic surface. The metallic surface acts as the secondary winding. When RF current is applied to the primary winding, the metallic surface will have RF current flow induced within it.
A need exists in the art for coupling RF energy to existing metallic structures, so as to allow the structures to be used as RF radiating surfaces. The RF energy must be coupled in a safe and efficient manner so as to maximize the radiation of RF power while minimizing the risk to people in the vicinity and the RF energy source.
It is an object of the present invention to provide an apparatus and method that combine RF transmission antenna technology with RF current injection probe technology in a heretofore unknown manner to eliminate conventional antennas by utilizing existing wires or surfaces of metallic structures. It is a further object of the present invention to provide for coupling of RF energy in a manner that maximizes the RF power radiated while minimizing the risks associated with such RF energy coupling.
The present invention eliminates the use of conventional antennas by taking advantage of the fact that any electrical conductor or surface being 0.1 wavelengths long or longer, when injected with significant current will radiate power.
The present invention accomplishes this goal by providing a transformer capable of effectively coupling transmitter power to a wire filament or surface, and creating current that will cause radiated power to emanate from the wire or surface.
The useful frequency range of some embodiments of the present invention has been demonstrated to be from 2 MHz to 1 GHz.
One of the preferred embodiments of the invention is a toroidal transformer with the primary connected to a transmitter, and the secondary consisting of any wire or metallic structure at least 0.1 wavelengths long. The secondary wire or metallic structure can be connected directly to a fastener located in the earth or to a metallic structure. No impedance termination or special treatment of the secondary is required. The primary is a winding wound around the toroidal magnetic core. This embodiment of the present invention provides the capability to use metallic structures, such as guy wires, flag poles, metal pipe, and architectural steel rebar, as transmitting antennas. The metallic structure must merely be positioned within the center of the toroidal transformer and RF energy will be coupled from the primary to the metallic structure.
Another preferred embodiment is that of a magnetic core, consisting of a half toroid or straight rectangular bar of magnetic material with its primary winding wound around the magnetic core (in a solenoid fashion). The secondary winding consists of a metallic surface, which can be a portion of a vehicle, ship superstructure, sail boat stay or mast or other metallic architectural structure. These surfaces, when injected with a current, will radiate an RF field directly proportional to the current injected. The source of the RF currents, originates from an RF transmitter output being connected and impedance matched to the primary winding of the transformer. When the ends of the toroidal core half or rectangular bar are placed in close proximity to the surface, the coupling efficiency is increased, generating more current in the surface. This embodiment of the present invention allows for nearly any conductive surface, such as metal surfaces present on a vehicle, ship, or building, to be used as a transmitting antenna. The magnetic core must be placed adjacent the conductive surface and oriented appropriately, and RF energy will be coupled from the primary to the conductive surface.